GB2101160A

GB2101160A - Forming an electrolyte/electrode assembly

Info

Publication number: GB2101160A
Application number: GB08216501A
Authority: GB
Inventors: Michael J Covitch; Mark F Smith; Leo L Benezra
Original assignee: Diamond Shamrock Corp
Current assignee: Diamond Shamrock Corp
Priority date: 1981-06-26
Filing date: 1982-06-07
Publication date: 1983-01-12
Also published as: CA1185126A; JPS586985A; DE3223830A1; US4386987A

Description

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GB 2 101 160 A

1

SPECIFICATION

Electrolytic cell membrane/SPE formation by solution coating

5 Field of the invention

This invention relates to batteries, fuel cells and electrochemical cells, and particularly to separators utilized in such cells. More specifically, this invention relates to solid polymeric electrolyte cell separators, polymeric cell membranes and methods for fabricating and attaching electrodes to such solid polymeric electrolytes and polymeric membranes for use in electrochemical cells.

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Background of the invention

The use of separator between an anode and cathode in batteries, fuel cells, and electrochemical cells is known. In the past, these separators have been generally porous separators, such as asbestos diaphragms, used to separate reacting chemistry within the cell. Particularly, for example, in diaphragm chlorine 15 generating cells, such a separator functions to restrain back migration of OH- radicals from a cell compartment containing the cathode to a cell compartment containing the anode. A restriction upon OH-back migration has been found to significantly decrease current inefficiencies associated with a reaction of the OH" radical at the anode releasing oxygen.

More recently separators based upon an ion exchange copolymer have found increasing application in 20 batteries, fuel cells, and electrochemical cells. One copolymeric ion exchange material finding particular acceptance in electrochemical cells such as chlorine generation cells has been fluorocarbon vinyl ether copolymers known generally as perfluorocarbons and marketed by E.I. duPont under the name NAFION®.

These so-called perfluorcarbons are generally copolymers of two monomers with one momomer being selected from a group including vinyl fluoride, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, 25 chlorotrifluoroethylene, perfluoro(alkylvinyl ether), tetrafluoroethylene and mixtures thereof.

The second monomer is selected from a group of monomers containing an S02F or sulfonyl fluoride group. Examples of such second monomers can be generically represented by the formula CF2=CFRiS02F. R-i in the generic formula is a bifunctional perfluorinated radical comprising one to eight carbon atoms. One restraint upon the generic formula is a general requirement for the presence of at least one fluorine atom on 30 the carbon atom adjacent the -S02F, particularly where the —S02F group exists as the —(—S02NH)mQform. In this form, Q can be hydrogen or an alkali or alkaline earth metal cation and m is the valence of Q. The F?i generic formula portion can be of any suitable or conventional configuration, but it has been found preferably that the vinyl radical comonomer join the Ft, group through an ether linkage.

Typical sulfonyl fluoride containing monomers are set forth in U.S. Patent NOs. 3,282,875; 3,041,317; 35 3,560,568; 3,718,627 and methods of preparation of intermediate perfluorocarbon copolymers are set forth in U.S. Patent Nos. 3,041,317; 2,393,967; 2,559,752 and 2,593,583. These perfluorocarbons generally have pendant S02F based functional groups.

Chlorine cells equipped with separators fabricated from perfluorocarbon copolymers have been utilized to produce a somewhat concentrated caustic product containing quite low residual salt levels. Perfluorocarbon 40 copolymers made from perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) comonomer have found particular acceptance in Cl2 cells.

Many chlorine cells use a sodium chloride brine feedstock. One drawback to the use in such cells of perfluorocarbon separators having pendant sulfonyl fluoride based functional groups has been a relatively low resistance in desirably thin separators to back migration of caustic formed in these cells, including OH-45 radicals, from the cathode to the anode compartment. This back migration contributes to a lower current utilization efficiency in operating the cell since the OH- radicals react at the anode to produce oxygen. Recently, it has been found that if pendant sulfonyl fluoride based cationic exchange groups adjacent one separator surface were converted to pendant carbonyl groups, the back migration of OH- radicals in such Cl2 cells would be significantly reduced. Conversion of sulfonyl fluoride groups to carboxylate groups is 50 discussed in U.S. Patent NO. 4,151,053.

Presently, perfluorocarbon separators are generally fabricated by forming a thin membrane-like sheet under heat and pressure from one of the intermediate copolymers previously described. The ionic exchange capability of the copolymeric membrane is then activated by saponification with a suitable or conventional compound such as strong caustic. Generally, such membranes are between 0.5 mil and 150 mil in thickness. 55 Reinforced perfluorocarbon membranes have been fabricated, for example, as shown in U.S. Patent No. 3,925,135.

Notwithstanding the use of such membrane separators, a remaining electrical power inefficiency in many batteries, fuel cells and electrochemical cells has been associated with a voltage drop between the cell anode and cathode attributable to passage of the electrical current through one or more electrolytes separating 60 these electrodes remotely positioned on opposite sides of the cell separator.

Recent proposals have physically sandwiched a perfluorocarbon membrane between an anode-cathode pair. The membrane in such sandwich cell construction functions as an electrolyte between the anode-cathode pair, and the term solid polymer electrolyte (SPE) cell has come to be associated with such cells, the membrane being a solid polymer electrolyte. Typical sandwich SPE cells are described in U.S. 65 Patent Nos. 4,144,301; 4,057,479; 4,056,452 and 4,039,409.

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2 GB 2 101 160 A

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At least one difficulty has surfaced in the preparation of SPE sandwiches employing reticulate electrode structures. Generally these sandwich SPE electrode assemblies have been prepared by pressing a generally rectilinear electrode into one surface of a perfluorocarbon copolymeric membrane. In some instances, a second similar electrode is simultaneously or subsequently pressed into the obverse membrane surface. To 5 avoid heat damage to the copolymeric membrane, considerably pressure, often as high as 6000 psi is required to embed the electrode firmly in the membrane. For reasons related to reticulate electrode structural configuration, such pressure is generally required to be applied simultaneously over the entire electrode area, requiring a press of considerable proportions when preparing a commercial scale SPE electrode. As yet, the solution coating of such electrodes with perfluorocarbon copolymer has not been 10 feasible principally dueto difficulties in developing a suitable solventfor perfluorocarbon copolymer.

The use of alcohols to solvate particularly low equivalent weight perfluorocarbon copolymers is known. However, as yet, proposals for formation of at least partially solvated perfluorocarbon dispersions and for solution coating electrodes with the copolymer perfluorocarbon where the perfluorocarbon is of a relatively elevated equivalent weight desirable in, for example, chlorine cells, have not proven satisfactory. 15 Dissatisfaction has been at least partly dueto a lack of suitable techniques for dispersing and/or solvating these higher equivalent weight perfluorocarbons.

Disclosure of the invention The present invention provides a method for forming an integral electrolytic cell membrane and solid 20 polymer electrolyte (SPE) while concurrently attaching an electrode. A cell membrane that is integral with a solid polymer electrolyte and carried by a cell electrode results from the method.

A device made in accordance with the instant invention includes an electrode structure suitable for use in a fuel cell, battery, electrochemical cell orthe like. This electrode structure includes inerstices. A portion of the electrode structure is coated with a copolymeric perfluorocarbon, the perfluorocarbon coating bridging the 25 interstices f the electrode structure. The thickness and continuity of the copolymeric perfluorocarbon bridging the interstices should be contiguous and sufficiently thick to preclude free movement of liquids within the cell from one side of the coated electrode structure to the other. More than one coating of one or more perfluorocarbon copolymer may be applied whereby the integral membrane and SPE possess more than one desirable functional group attribute of the perfluorocarbon copolymer.

30 A solid polymer electrolyte-electrode of the instant invention is prepared by a process when a selected perfluorocarbon copolymer is dispersed in at least partially solvating dispersion media. A desired electrode structure is then at least partially coated with the dispersion, the dispersion bridging the interstices. The dispersion media is removed following coating. Repeated cycles of coating and subsequent removal of the dispersion media may be desirable in achieving an integral membrane and SPE having desired polymeric 35 functional group properties and/or to achieve a desired thickness.

In certain preferred embodiments, the electrode structure can include surface portions comprising one or more electrocatalytic compounds. In forming solid polymer electrolyte-electrodes using such electrode structures, it is desirable that these electrocatalytically active surfaces not be coated accomplished by a method such as masking the electrocatalytic surface portions priorto coating.

40 Where the electrode structure is coated with the dispersion to an extent providing a coating over a greater area of the surface of the electrode structure than is desired, in certain preferred embodiments, the coating covering the undesirable electrode structure suface areas can be removed.

The above and other features and advantages of the invention will become apparent from the following detailed description of the invention made with reference to the accompanying drawings which form a part 45 of the specification.

Brief description of the drawings

Figure 1 is an elevational view of the solid polymer electrolyte-electrode of this invention viewed from the coated side.

50 Figure 2 is a partial side elevational cross sectional view of the solid polymer electrolyte-electrode of the instant invention.

Best embodiment of the invention

Referring to Figures 1 and 2, an integral membrane and solid polymer electrolyte-electrode is shown 55 generally at 10. The solid polymer electrolyte (SPE) electrode 10 is comprised of an electrode structure 15 and a polymer coating 20.

The electrode structure 15 is generally of reticulate form but equally may be of sintered metal or other suitable or conventional configuration. The electrode structure 15 includes interstices 25.

The polymer coating 20 coats generally one surface of the electrode structure 15 and bridges or blinds the 60 interstices 25. All interstices to be immersed in electrolyte contained in the electrochemical cell must be entirely blinded. The thickness of the coating, particularly that coating bridging the interstices, can be varied, but generally ranges between 0.5 and 150 mils and preferably ranges between 4 and 10 mils.

Where the SPE-electrode 10 is to be used as an anode, the surface 30 remaining uncoated can include an electrocatalytic surface portion 35. This electrocatalytic portion 35 includes at least one compound selected 65 from the group consisting of gold, silver and oxides of: iron, nickel, chromium, antimony, tin, cobalt, copper,

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GB 2 101 160 A

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lead, manganese, titanium, and a platinum group metal; the platinum group comprising platinum, palladium, osmium, iridium, rhodium and ruthenium.

The electrode structure 15 is made principally from a suiable or conventional substrate such as: Periodic Table Group IVA metals tin and lead; Periodic Table Group IB metals copper, silver and gold; Periodic Table 5 Group 8 metals cobalt, nickel, iron including stainless steels, ruthenium, rhodium, palladium, osmium, iridium and platinum; as well as manganese, chromium, vanadium, titanium, niobium, zirconium, bismuth, tantalum, aluminum and carbon. Where the SPE-electrode 10 is to function as an anode, the electro-catalytic compound is applied to the anode in any well-known manner.

The SPE electrode 10 can be employed in an electrolytic cell such as a sodium chloride rine based chlorine 10 generation cell. Where the electrode structure 15 is to function as an anode, it advantageously includes the electro-catalytic surface portion 35. Sodium chloride brine present in the cell generally at 37 reacts at the anode to release Cl2 and Na+ cations. The Na+ cations negotiate the membrane-SPE 20 carrying current between cell anode and cathode and are thereafter available for reaction at a cell cathode of suitable or conventional configuration. Alternately, the reticulate electrode structure can perform as a cathode whereby 15 sodium ions negotiating the coating 20 react to form caustic NaOH with hydroxyl ions liberated by the cathodic dissociation of water.

The SPE electrode 10 of the instant invention is prepared by at least partially coating the reticulate electrode struture within a dispersion of perfluorocarbon copolymer having pendant functional groups capable of being converted to ion exchange functional groups such as groups based upon or derived from 20 sulfonyl, carbonyl, or in some cases phosphoric functional groups. The coating can be accomplished in any suitable or conventional manner such as by dipping, spraying, brushing or with a roller. Following coating, the dispersing media for the perfluorocarbon copolymer is removed, usually by the application of gentle heat and, if desired, vacuum, or by leaching with a suitable or conventional light solvent such as acetone, 2-propanol or a halogenated hydrocarbon such as FREON® 113, a product of duPont. One or more coatings 25 may be required to provide a coating of desired thickness and one that effectiely blinds all the interstices of the electrode structure 15.

The copolymeric perfluorocarbon dispersed for use in coating the electrode structure is generally an intermediate copolymer having functional groups providing latent ion exchange capability later activated or an ion exchange activated copolymer. The intermedite polymer is prepared from at least two monomers that 30 include fluorine substituted sites. At least one of the monomers comes from a group that comprises vinyl fluoride, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro(alkyl vinyl ether), tetrafluoroethylene and mixtures thereof.

At least one of the monomers comes from a grouping having members with functional groups capable of imparting cationic exchange characteristics to the final copolymer. Monomers containing pendant sulfonic 35 acid, carboxylic acid or, in some cases phosphoric acid functional groups are typical examles. Condensation esters, amides or salts based upon the same functional groups can also be utilized. Additionally these second group monomers can include a functional group into which an ion exchange group can be readily introduced and would thereby include oxyacids, salts, or condensation esters of carbon, nitrogen, silicon, phosphorus, sulfur, chlorine, arsenic, selenium, ortellurium.

40 Among the preferred families of monomers in the second grouping are sulfonyl containing monomers containing the precursor functional group S02ForS03alkyl. Examples of members of such a family can be represented by the generic formulae of CF2=CFS02F and CF2=CFR-|S02F where R-i is a bifunctional perfluorinated radial comprising 2 to 8 carbon atoms.

The particular chemical content or structure of the perfluorinated radical linking the sulfonyl group to the 45 copolymer chain is not critical and may have fluorine, chlorine or hydrogen atoms attached to the carbon atom to which the sulfonyl group is attached, although the carbon atom to which the sulfonyl group is attached must also have at least one fluorine atom attached. If the sulfonyl group is attached directly to the chain, the carbon in the chain to which it is attached must have a fluorine atom attached to it. The R-i radical of the formula above can be either branched or unbranched, i.e., straight chained, and can have one or more 50 ether linkages. It is preferred that the vinyl radical in this group of sulfonyl fluoride containing comonomers to be joined to the Ri group through an ether linkage, i.e., that the comonomer by of the formula CF2=CF0RiS02F. Illustrative of such sulfonyl fluoride containing comonomers are:

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4 GB 2 101 160 A

cf2=cfocf2cf2so2f, cf2=cfocf2cfocf2cf2so2f,

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cf3

5 cf2=cfocf2cfocf2cfocf2cf2so2f, cf2=cfcf2cf2so2f,

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cf3 cf3

and

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cf2=cfocf2cfocf2cf2so2f cf2

15 0 11

cf3

20 The corresponding esters of the aforementioned sulfonyl fluorides are equally preferred.

While the preferred intermediate copolymers are perfluorocarbon, that is perfluorinated, others can be utilized where there is a fluorine atom attached to the carbon atom to which the sulfonyl group is attached. A highly preferred copolymer is one of tetrafluoroethylene and perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) comprising between 10 and 60 weight percent, and preferably between 25 and 40 weight percent, of 25 the latter monomers.

These perfluorinated copolymers may be prepared in any of a number of well-known manners such as is shown and described in U.S. Patent Nos. 3,041,317; 2,393,967; 2,559,752 and 2,593,583.

An intermediate copolymer is readily transformed into a copolymer containing ion exchange sites by conversion of the sulfonyl groups (-S02F or S03 alkyl) to the form S03X by saponification orthe like

30 wherein X is hydrogen, an alkaly metal, or an alkaline metal. The converted copolymer contains sulfonyl fluoride based ion exchange sites contained in side chains of the copolymer and attached to carbon atoms having at least one attached fluorine atom. Not all sulfonyl groups within the intermediate copolymer need be converted. The conversion may be accomplished in any suitable or customary manner such as is shown in U.S. Patent Nos. 3,770.547 and 3,784,399.

35 A coating 20 made from copolymeric perfluorocarbon having sulfonyl based cation exchange functional groups possesses a relatively low resistance to back migration of sodium hydroxide from cathodic areas of the cell 39 to the anodic cell areas 37, although such a membrane successfully resists back migration of other caustic compounds such as KOH. Where the sulfonyl fluoride group is at least partially converted to a sulfonamide by treating with propylamine orthe like, usefulness in a chlorine cell based upon NaCI 40 electrolysis may be improved.

In some preferred modes for carrying out the invention, the coating includes pendant carbonyl based functional groups. The pendant carbonyl based groups provide the copolymeric perfluorocarbon with significantly greater resistance to the migration of sodium hydroxide, but can also substantially reduce the rate of migration of sodium ions from the anode to the cathode.

45 Copolymeric perfluorocarbon having pendant carbonyl based cationic exchange functional groups can be prepared in any suitable or conventional manner such as in accordance with U.S. Patent No. 4,151,053 or Japanese Patent Application 52(1977)38486 or polymerized from a carbonyl functional group containing monomer derived from a sulfonyl group containing monomer by a method such as is shown in U.S. Patent NO. 4,151,053. Preferred carbonyl containing monomers include CF2=CF-0-CF2CF(CF3)0(CF2)2C00CH3 50 and CF2=CF-0-CF2CF(CF3)0CF2C00CH3.

Preferred copolymeric perfluorocarbons utilized in the instant invention therefore include carbonyl and/or sulfonyl based groups represented by the formula OCF2CF2X and/or 0CF2CF2Y—B-YCF2CF20

wherein X is sulfonyl fluoride (S02F) carbonyl fluoride (C02F) sulfonate methyl ester (SOOCH3) carboxylate methyl ester (COOCH3) ionic carboxylate (COO~Z+) or ionic sulfonate (S03~Z+), Y is sulfonyl or carbonyl

55 (-SO CO - ), and B is a cross-linking structure such as -O—, —O—O—, —S-S—, and di and poly amines of the form NH(CF1R2)XNH2 where R1f R2 are selected from short chain alkanes, alkenes, hydrogen, and amine groups and Z is hydrogen, an alkali metal such as lithium, cesium, rubidium, potassium and sodium or an alkaline earth such as barium, beryllium, magnesium, calcium, strontium and radium or a quaternary ammonium ion. B forms of other than -O- display relatively low cation exchange functionality, however. 60 Generally, sulfonyl, carbonyl, sulfonate and carboxylate esters and sulfonyl and carbonyl based amide forms of the perfluorocarbon copolymer are readily converted to a salt form by treatment with a strong alkali such as NaOH.

The equivalent weight range of the copolymer intermediate used in preparing the membrane 15 is important. Where lower equivalent weight copolymers are utilized, the membrane can be subject to 65 destrutive attack such as disolution in cell chemistry. When as excessively elevated equivalent weight

GB2 101 160 A

copolymer is utilized, the membrane may not pass cations sufficiently readily resulting in an unacceptably low electrical efficiency in operating the cell. It has been found that copolymer inermediate equivalent weights should preferably range between about 1000 and 1500 for the sulfonyl based membrane materials and between about 900 and 1500 for the carbonyl based membrane materials.

5 The electrocatalytic anode substance can be applied as a component of one or more coatings to an 5

electrode structure. When applied to an electrode structure, the electrocatalytic compound can be applied directly over an electrode substrate, generally a valve metal such as titanium orthe like well known in the art,

or it may be applied over a primary coating first applied to the substrate of types also well known in the art. The electrocatalytic coating is generally applied to electrode structure portions and intended to be coated by 10 the copolymeric perfluorocarbon. Coverage of the electrode with the electrocatalytic substance is usually 10 constrained to surfaces not coated with the copolymerto avoid a separation of the coating from the electrode structure 15 that would accompany generation of chlorine gas at copolymer coated electrode structure surfaces. For the same reasons, it is necessary to season or render inactive those portions of the electrode substrate structure 15 to be coated by the copolymer. Seasoning avoids generation of chlorine gas 15 beneath the coating adjacent the electrode structure 15 that would cause a separation of the coating. Desired 15 portions of the electrode structure 15 can be rendered inactive by the brief actual generation of chlorine using the electrode structure before copolymer coating.

Perfluorocarbon copolymer is dispersed in any suitable or conventional manner. Preferably relatively finely divided particles of the copolymer are used to form the dispersion. The particles are dispersed in a 20 dispersion media that preferably has significant capability for solvating the perfluorocarbon copolymer 20 particles. A variety of solvating dispersers have been discovered for use with the perfluorocarbon copolymers; these suitable solvating dispersers are tabulated in Table I and coordinated with the copolymer pendant functional groups with which they have been found to be an effective dispersion medium. Since one or more of the dispersers may be used together in preparing a perfluorocarbon dispersion, as well as one or 25 more of the dispersers suitably diluted, the term dispersion media is used to refer to a suitable or 25

conventional solvating dispersion agent having at least one solvating dispersion medium.

TABLE I

30 Solvent cross reference to perfluorocarbon copolymer 30

containing various pendant functional groups

Solvent Functional group

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so2f coo~z+

COO(ester)

S03"Z+ 35

Halocarbon Oil

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perfluorooctanoic x

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perfluorodecanoic acid

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perfluorotributylamine

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FC-70 available from 3M

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(perfluorotrialkylamine)

perfluoro-1-methyldecalin

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decafluorobiphenyl

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pentafluorophenol

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pentafluorobenzoic acid

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N-butylacetamide

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tetrahydrothiophene-1,1-dioxide

(tetramethylene sulfone, Sulfolane®)

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N,N-dimethyl acetamide

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N,N-diethyl acetamide

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N,N-dimethyl propionamide

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N,N-dibutylformamide

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N,N-dipropylacetamide

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N,N-dimethyl formamide

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1-methyl-2-pyrrolidinone

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diethylene glycol

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ethylacetamidoacetate

X

60 Z is any alkaly or alkaline earth metal or a quaternary ammonium ion having attached hydrogen, alkyl, 60 substituted alkyl, aromatic, or cyclic hydrocarbon. Halocarbon Oil is commercially marketed oligomer of chlorotrifluoroethylene.

Certain of the solvating dispersion media functions more effectively with perfluorocarbon copolymer 65 having particular metal ions associated with the functional group. For example, N-butylacetamide functions 65

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well with the groups COOLi and S03Ca. Sulfolane and N,N-dipropylacetamide function well with So3Na functionality.

It is believed that other suitable or conventional perhalogenated compounds like perfluorotrialkyl amines can be used for at least partially solvating the S02F or carboxylate ester forms of perfluorocarbon 5 copolymer. It is believed that other suitable or conventional strongly polar compounds can be used for solvating the ionic sulfonate and carboxylate forms of the perfluorocarbon copolymer.

In at least partially solvating the perfluorocarbon polymers, it is frequently found necessary to heat a blend of the dispersion media and the relatively finely divided perfluorocarbon to a temperature between about 50°C and 250°C but not in excess of the boiling point for the resulting dispersion. Depending upon the 10 solvating dispersion medium, a solution of between about 5 and 25 weight percent results. It is not necessary that the perfluorocarbon be dissolved completely in orderto form a suitable electrode coating. It is important that perfluorocarbon particles remaining unsolvated be relatively small to produce a smooth void free coating particularly in bridging the interstices. In one alternate technique, the dispersion is heated to at least approach complete solvation and then cooled to from a gel having particles of approximately the size 15 desired to form the coating. The particle size is controllable using either of mechanical or ultrasonic disruption of the gelatinous dispersion.

Referring to Table I, it may be seen that various solvents have a particularly favorable effect upon only perfluorocarbon copolymers having certain functional groups. An SPE coated electrode 10 containing perfluorocarbon having functional groups of a first type can be at least partially solvent welded to a 20 perfluorocarbon coated electrode having functional groups of a second type; however, conversion of one or both types of functional groups may be necessary to achieve solvent compatability. Particularly, hydrolysis and substitution of metal ions ionically bonded to the functional group can provide a relatively simple tool for coordinating functional groups and solvents. However, other methods such as the use of SF4to reform sulfonyl fluoride functional groups from derivatives of sulfonyl fluoride are also available.

25 One simple method for constraining dispersion from coating electrocatalytic portions 35 of the electrode structure 15 is to mask those electrocatalytic portions 35 while coating the electrode structure 15 with the dispersion. A reticulate electrode can be effectively masked by pressing the electrode structure into a sheet of aluminum foil covering a sheet of a resinous material that relatively readily undergoes cold flow. Cold flow in the relatively slow flowing of a material away from an object being pressed into the material. 30 Particularly, an E.I. duPont product, TEFLON®, in the form of fluoroethylene polymer (FEP) or polytetrafluoroethylene (PTFE) has been found to be particularly useful for use as the resinous sheet. As the electrode structure is pressed into the aluminum foil, the TEFLON supporting the foil cold flows from beneath the electrode structure towards the interstices of the electrode structure. The foil is urged by the cold flowing TEFLON to conform closely to contours of the electrode structure including portions of the 35 electrode structure surrounding the interstices. Where the surface of the electrode structure pressed into the foil includes electrocatalytic portions, the electrocatalytic portion can thereby be effectively masked.

Where an entire electrode structure has been immersed in dispersed copolymer and thereby coated, it is desirableto expose some portion of the electrode structure. Selective removal of the coating can be accomplished by any suitable or conventional method such as grinding, scarifying, cutting orthe like. 40 Where desired, ion exchange functional groups adjacent one coating surface can be converted from, for example, sulfoyl based groups to carboxylate based groups. Conversion, such as by methods shown in U.S. Patent 4,151,053 can provide a carboxylate based layer 40 in the coating that assists in resisting sodium hydroxide backmigration from the cell cathode to the cell anode while retaining a desirable sulfonyl based layer 45 more freely permeable to sodium ions seeking to migrate to the cell cathode.

45 In a preferred alternate, one or more coatings of a copolymer containing a particular functional group is applied to the electrode 15 followed byoneormore coatings of copolymer containing a second functional group. Where the copolymers are mutually soluble in dispersing media used for dispersing the second copolymer, a solvent bond between the coating applications is established by which they become coadhered.

50 In one typical example, perfluorocarbon containing pendant sulfonyl fluoride groups is applied to unmeshed portions of an electrode to be used as an anode. The sulfonyl fluoride group containing copolymer is dispersed in Halocarbon Oil, perfluorodecanoic acid or perfluorooctanoic acid.

After establishing a contiguous coating of desired thickness, a further coating of a second copolymer containing pendant methyl carboxylate ester groups is applied overthe original coating again using 55 Halocarbon Oil, perfluorooctanoic acid or perfluorodecanoic acid as the dispersion media.

Functional groups in both copolymers are then saponified using KOH to yield an integral SPE and membrane having sulfonyl based cationic exchange groups opposing the anode, and carbonyl based functional groups opposing a cathode utilized in conjunction with the anode in a cell.

Further, a cathode coated on one surface with a functional copolymeric fluorocarbon containing pendant 60 first functional groups can be solvent adhered to an anode having a perfluorocarbon coating containing pendant second functional groups, or each can be solvent adhered to an intervening perfluorocarbon copolymeric film. Heat and/or pressure may be necessary to assure acceptable coadherence using solvents, but under extremes, of temperature and pressure, such as 2000-6000 psig and temperatures in excess of 100°C+ a solvent may be unnecessary for coadherence.

65 The following examples are offered to illustrate furtherthe invention.

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Example /

Perfluorocarbon copolymer having pendant S02F functional groups and polymerized from polytetrafluoroethylene and perfluoro(3,6-dioxa-4-methyl-7-octenesulfonylfluoride) and having an equivalent weight of about 1100 was dissolved in hot (240°C) Halocarbon Oil to yield a 12 percent (weight) solution-dispersion.

5 A titanium expanded mesh, 10 Ti 14-3/0 (read as titanium mesh having a wire thickness of 10 mils, a wire width of 14 mils, a mesh opening having a long dimension of about 1/8 inch and a short dimension of about 50 mils) is coated on one side with an electrocatalytic coating such as is described and shown in U.S. Patent 3,751,296. A sheet of aluminum foil was sandwiched between the electrocatalytic surface and a sheet of TEFLON and the electrode pressed into the foil and TEFLON.

10 The mesh was then mounted upon a frame and immersed in the dispersion, withdrawn and the Halocarbon Oil removed by extraction using FREON 113. Immersion and extraction were repeated. The mesh was demounted from the frame and hydrolyzed in weak KOH for 96 hours at room temperature which served also to leach the aluminum foil from the mesh. A 4 mil contiguous cationic exchange coating resulted on the mesh.

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Example II

A procedure identical to that of Example I was performed using a sheet of porous titanium, made by sintering titanium particles coated with an electrocatalytic coating as in Example I. A contiguous 4 mil coating resulted upon the sheet.

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Example III

A titanium mesh 5Ti 7-3/0 electrocatalytically coated as in Example I and a nickel mesh 5 Ni 7-3/0 were each masked on one side using aluminum foil and TEFLON under pressure in accordance with Example I. The meshes were installed in a frame and coated in accordance with Example I. After removal of the

25 dispersion media, the coated surfaces were then aligned with a perfluorocarbon film between them and pressed at 180°C and 2000 psig until each coadhered to the film. The resulting composite film was a 23 mil thickness including both electrodes.

The laminated electrode structure was saponified in weak KOH.

30 Example IV

Sulfonyl fluoride functional groups in the coatings of Examples I, II and III are converted in part by n-propyl amine to sulfonamide functionality before saponfication. The resulting coating provides superior chlorine cell performance to coatings including only saponified sulfonyl fluoride functional groups.

While a preferred embodiment of the invention has been described in detail, it will be apparent that

35 various modifications or alterations may be made therein without departing from the scope of the invention as set forth in the appended claims.

Claims

40 1. A method of forming an electrolyte/electrode assembly, for use in an electrochemical cell, which comprises applying, to an electrode structure which includes interstices, a dispersion comprising a copolymeric perfluorocarbon having sulphonyl, carbonyl and/or phosphorus-based pendant functional groups in a solvating dispersion medium, whereby the dispersion at least partially coats the electrode structure and bridges the interstices, and removing the dispersion medium.

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2. A method according to claim 1, wherein at least one coating of a further copolymeric compound having a second pendant functional group is applied to the coated assembly.
3. A method according to claim 1 or 2, wherein the dispersion is applied to the electrode structure by immersion.
4. A method according to any of claims 1 to 3, wherein one or more portions of the electrode structure

50 is/are masked prior to applying the dispersion.
5. A method according to claim 4, wherein the masking is applied to the electrocatalytic surface portion or portions of a reticulate electrode structure having at least one elecrocatalytic compound thereon, the dispersion is applied and the medium is removed one or more times until a coating of desired thickness is produced upon the electrode structure completely bridging the interstices and the masking is then removed.

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6. A method according to claim 5, wherein the masking is effected by placing a sheet of a masking material over a sheet of a resinous material capable of undergoing cold flow, placing the electrode structure upon the masking material sheet with the electrocatalytic surface portion or portions opposing the sheet and pressing the electrode structure into the masking material sheet until the resinous material undergoes cold flow, thereby supporting the masking material sheet as it conforms to the electrocatalytic surface portion or

60 portions of the reticulate electrode structure.
7. A method according to claim 6, wherein the masking material comprises a sheet of aluminium foil, which is placed over a sheet of polytetrafluoroethylene (TEFLON), the reticulate electrode structure is placed upon the aluminium foil with the electrocatalytic surface portion opposing the aluminium foil and, after the pressing and coating steps, the aluminium foil and polytetrafluoroethylene sheet are removed.

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8. A method according to any of claims 1 to 3, wherein the coating is removed from one or more portions

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10

15

20

25

30

35

40

45

50

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GB 2 101 160 A

of the electrode structure afterthe steps of applying the dispersion and removing the medium have been completed.
9. A method according to claim 8, wherein the dispersion is applied and the medium is removed one or more times from a reticulate electrode structure, until a coating of desired thicknes is produced upon the

5 electrode structure and interstices in the electrode structure have been bridged and at least one portion of 5 the coating is then removed so as to expose a portion or portions of the reticulate electrode structure.
10. A method according to any of claims 6 to 9, wherein the reticulate electrode structure comprises nickel.
11. A method according to any preceding claim, wherein the steps of applying the dispersion and

10 removing the medium are effected more than once. 10
12. A method according to any of claims 5 to 11, wherein the electrocatalytic compound or compounds is/are selected from oxides of manganese, tin, antimony, titanium, vanadium and platinum group metals.
13. A method according to any preceding claim, wherein the perfluorocarbon is a copolymer of at least one fluorinated vinyl compound and at least one monomer having the structure:

15 15

CF2 = CF, CF2 = CF, and CF2 = CF ! I I

X Rt O

i I

20 X R, 20

I

X

wherein R1 is a bifunctional perfluorinated radical containing from 2 to 8 carbon atoms which carbon atoms

25 can be at least once interrupted by one or more oxygen atoms and X is selected from sulphonyl fluoride, 25 carboxyl fluoride, sulphonate esters, carboxylate esters and saponification products of sulphonyl fluoride and carboxyl fluoride.
14. A method according to any preceding claim, wherein the dispersion mediumis selected from Halocarbon Oil, perfluorooctanoic acid, perfluorodecanoic acid, perfluorotributylamine, perfluoro-1-

30 methyldecalin, decafluorobiphenol, pentafluorophenol, pentafluorobenzoic acid, N-butylacetamiede, tet- 30 rahydrothiophene-1,1-dioxide (tetramethylene sulphone), N,N-diethylacetamide, N,N-dimethylpropionamide, N,N-dibutylformamide, N,N-dimethylacetamide. FC-70 and dipropylamide.
15. A method according to any preceding claim, wherein coated portions of a second electrode assembly are adhered to the electrode assembly by means of at least one of heat, pressure and solvent.

35
16. A method according to claim 1, substantially as hereindescribed. 35
17. An electrolyte electrode assembly when made by a method according to any preceding claim.
18. An electrochemical cell, when containing an electrolyte/electrode assembly according to claim 17.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1983. Published by The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.